1. Field of the Invention
The invention relates to heat-curable molding compositions based on bismaleimide resins. More particularly, the invention relates to improved bismaleimide resin compositions having improved toughness without sacrificing thermal stability.
2. Background of the Invention
Bismaleimide resins, which also contain aromatic diamines as comonomers as disclosed in Federal Republic of Germany Patent 1 770 867 demonstrate excellent mechanical and electrical properties after thermal curing at temperatures in excess of 200.degree. C. However, the monomers have limited solubility in ordinary solvents and the cured products are very brittle. Maleimide resins containing polyphenols as comonomers according to Federal Republic of Germany published application 2 459 925 demonstrate insufficient toughness.
An improvement in toughness may be achieved by using binuclear allylphenols or allylphenol esters, as described, for example, in U.S. Pat. No. 4,100,140. However, these comonomers show a relatively low fusion viscosity creating difficulties when impregnating reinforcing fibers as is commonly done in the preparation of fiber-reinforced prepregs. As a result of the relatively large proportion of aliphatic structural elements, these resins do not possess sufficient thermal-oxidative stability for certain applications.